In recent years, the Internet of Vehicles attracts increasingly more attention from people. In the Internet of Vehicles, vehicle to vehicle (V2V) communication or communication between a vehicle and a roadside unit (RSU) is used to improve road traffic safety and reliability and enhance traffic efficiency. In the Internet of Vehicles, to ensure vehicle driving safety, status information needs to be periodically exchanged between vehicles. In different standards or specifications, the information has different names, such as a basic safety message (BSM) and a cooperative awareness message (CAM) that are collectively referred to as a periodic status message (PSM) herein. The PSM may be understood as a “heartbeat packet” of a vehicle, includes vehicle information such as a location, a speed, and a status, and is broadcast to surrounding vehicles in a single-hop manner at a specific frequency. For example, the frequency may be 10 Hz. User equipment in the Internet of Vehicles moves at an extremely high speed, and therefore, the PSM has an extremely high requirement for a service delay.
The Institution of Electrical and Electronics Engineers (IEEE) 802.11p (also referred to as WAVE, Wireless Access in the Vehicular Environment) standard is a communications protocol extended from the IEEE 802.11 standard, and is mainly used for vehicle electronics wireless communication. The IEEE 802.11p standard is essentially extension of the IEEE 802.11, and meets related application in an intelligent transportation system (ITS), and the application includes data exchange between vehicles at a high speed, and data exchange between a vehicle and an ITS roadside infrastructure. The IEEE 802.11p standard has the following advantages: easy deployment, low costs, and a mature technology, and is applicable to vehicle to vehicle transmission. However, the standard also has corresponding disadvantages: When there are a large quantity of vehicles, resources are prone to conflict, and consequently, system performance becomes worse, a delay is uncontrollable, quality of service (QoS) cannot be ensured, a transmission distance is limited, and costs of deploying a large quantity of roadside units (RSU) are extremely high. Because of these disadvantages, a problem that the PSM has a high delay requirement in the Internet of Vehicles cannot be resolved by using the IEEE 802.11p standard.
The foregoing problem arouses people to undertake researches on assisting vehicle to vehicle communication by using an existing cellular network. Currently, 2G/3G/4G technologies are used for cellular communication. A Long Term Evolution (LTE) technology used in a 4G system has advantages such as a high rate, a low delay, a large coverage area, and supporting a high-speed mobile terminal. In vehicle to vehicle communication in the cellular network, a central scheduler such as an evolved NodeB (eNodeB, eNB) may be fully used to dynamically schedule transmission resources, so that a communication conflict probability is reduced, and a problem of an uncontrollable delay is resolved.
In an existing LTE system, if UE needs to transmit data, in an initial transmission process, a basic procedure is shown in FIG. 1. First, a primary synchronization signal/secondary synchronization signal (PSS/SSS) needs to be detected to perform downlink synchronization, and cell identification (ID) is obtained. Then, a master information block/system information block (MIB/SIB) is detected to obtain cell configuration information. Subsequently, the UE initiates an uplink random access process to perform uplink synchronization, obtains uplink resource allocation information by sending a scheduling request/buffer status report (SR/BSR), and finally, transmits the data by using an allocated resource. In the foregoing initial transmission process, because the UE needs to initiate the uplink random access process, an extremely large delay is caused. For example, the delay is approximately 50 to 100 ms. In addition, an SR/BSR transmission process also causes an extra delay. Consequently, the initial transmission process in the LTE system cannot meet a delay requirement in an Internet of Vehicles system.
In addition, in the existing LTE system, if the UE is in a connected state, and moves from one cell to another cell, cell handover occurs. A signaling procedure of the cell handover is shown in FIG. 2, and a handover process is as follows:
0. When a connection is set up or during last update of a timing advance (TA), context about UE roaming and an access restriction is provided to a source eNB.
1. The source eNB instructs, according to information about the UE roaming and the access restriction, the UE to perform physical layer measurement.
2. The UE reports a measurement report to the source eNB.
3. The source eNB determines, according to measurement report information and radio resource management (RRM) information, whether the UE needs to hand over a cell.
4. The source eNB initiates a handover request to a target eNB, and transmits, to the target eNB, information required for the handover.
5. The target eNB determines, according to received quality of service information, whether to allow the UE to perform the handover.
6. The target eNB sends handover acknowledge information to the source eNB to allow the UE to hand over the cell.
7. The source eNB sends a radio resource control connection reconfiguration message RRCConnectionReconfiguration to the UE.
8. The source eNB sends a serial number status transfer (SN STATUS TRANSFER) message to the target eNB.
9. After receiving the radio resource control connection reconfiguration message RRCConnectionReconfiguration, the UE synchronizes with the target eNB, and accesses a target cell by initiating a random access procedure RACH.
10. The target eNB transmits timing advance information and resource allocation information to the UE.
11. After the UE successfully accesses the target cell, the UE sends a radio resource control connection reconfiguration complete message RRCConnectionReconfigurationComplete and a BSR to the target cell, to indicate that the UE has completed a handover process and the target eNB can transmit data to the UE.
12. The target eNB transmits a path switch request PATH SWITCH REQUEST message to a mobility management entity MME, to notify the MME that the UE has changed the cell.
13. The MME transmits a modify bearer request MODIFY BEARER REQUEST message to a serving gateway.
14. The serving gateway switches a downlink data path to a target cell side.
15. The serving gateway transmits a modify bearer response MODIFY BEARER RESPONSE to the mobility management entity MME.
16. The MME responds to the path switch request PATH SWITCH REQUEST message by transmitting a path switch request acknowledge PATH SWITCH REQUEST ACKNOWLEDGE message.
17. After receiving the path switch request acknowledge PATH SWITCH REQUEST ACKNOWLEDGE message transmitted by the MME, the target eNB notifies, by transmitting a UE context release (UE CONTEXT RELEASE) message to the source eNB, the source eNB that the UE has successfully switched the cell, and triggers the source eNB to release a resource.
18. After receiving the UE context release message, the source eNB releases radio and control plane resources related to UE context.
In the cell handover process, the UE can transmit a service only after accessing the target cell by initiating the random access process, and obtaining the resource allocation information of the target cell. This process causes an extremely large delay. The foregoing handover process includes a large quantity of signaling overheads including signaling exchange between the UE and the source and target eNBs, signaling exchange between the source eNB and the target eNB, and the like. When the foregoing handover process is used in the Internet of Vehicles system to hand over a cell, because of a high-speed moving feature of a vehicle, cells are handed over frequently. Consequently, there is a relatively large handover delay and relatively high signaling overheads in the system.